The present invention relates to a multilayered, high density cellulose and superabsorber composite material useful for the preparation of superabsorbent cellulose fluff suitable for use in disposable absorbent articles such as infant diapers, feminine hygiene pads and adult incontinence pads.
Recently, the demand for thinner and more comfortable absorbent articles, such as infant diapers, feminine hygiene pads, adult incontinence pads, and the like, has increased. The materials which are most widely used for the absorbent core of such absorbent articles are cellulose fibers and superabsorbent polymers. Superabsorbent polymers (hereinafter xe2x80x9csuperabsorbersxe2x80x9d) developed recently are capable of absorbing many times their own weight of liquid. Superabsorbers have been used to increase the absorbency of absorbent products such as infant diapers, feminine hygiene pads and adult incontinence pads.
When manufacturing absorbent cores, cellulose fibers, generated e.g., by hammermilling a cellulose pulp board, and superabsorbent polymers are typically introduced into the core by introducing them separately into the pad forming unit of a converting machine. The superabsorbent polymers are often provided in the form of granules, powders or fibers. Superabsorbent particles are well-known in the art and are described, e.g., in U.S. Pat. Re. 32,649 and U.S. Pat. No. 4,102,340. One art-recognized problem with introducing the cellulose and superabsorber into the absorbent core as separate materials is the difficulty in handling the superabsorber. Separately added superabsorber creates dust, which complicates maintenance and reduces the efficiency of the absorbent product pad converting machine. Further, superabsorber can attach to the outside of the absorbent products and cause product quality issues with the consumer because of its grittiness and hygroscopicity. Superabsorber dust and free particle production also results in superabsorber waste and attendant economic inefficiency.
Most methods to contain the superabsorber particles in absorbent fluff are directed at the machine design of the converter. A method for immobilization of superabsorber is disclosed in U.S. Pat. No. 4,444,830, where an absorbent polymer solution is coated on base fluffing material to form a film, and the coated fluffing material is dried, disintegrated and mechanically worked into a fibrous fluff matrix which contains absorbent polymer platelets distributed throughout the matrix. Fluff produced from such superabsorbent film composite materials have reduced absorbent capacity relative to fluff containing superabsorber in particulate form, because of a xe2x80x9cgel blockingxe2x80x9d phenomenon. Gel blocking occurs in fluff made from base fluffing materials coated with superabsorber film because the superabsorber film (as opposed to particles) has a very low permeability. As a result, fluid to be absorbed does not have access to the full absorbent core produced by disintegrating the superabsorber film-coated base fluffing material . The interaction of the fluid and the film platelets causes the superabsorber to swell in localized regions of the core. The fluid is then blocked by the superabsorber gel that is locally formed, and liquid pooling, as opposed to absorption, occurs in the core. This results in large portions of the core being unused, and of failure of the absorbent core.
U.S. Pat. No. 4,424,247 discloses composite laminates made of superabsorber bonded to a wicking substrate, where the superabsorber and the substrate are present in a 1:1 ratio. The laminates can be disintegrated into an absorbent fluff material suitable for use as an absorbent core. The resulting core material, however, has low levels of superabsorber. EP 359 615 discloses a superabsorbent fiber structure comprising absorbent cellulose fibers and solid absorbent particles formed by laying a preformed web over the absorbent. The resulting density of this structure must be low in order to prevent crushing of the superabsorber, which results in unsatisfactory absorbency properties. The structure described by EP 359 615 is disclosed to be useful as an absorbent core per se, as opposed to being useful as a material for subsequent disintegration, e.g. by hammermilling, in the preparation of an absorbent fluff core material.
The current state of the art is such that absorbent cores for such absorbent articles are produced with about 40 to 50% by weight superabsorber. It would be advantageous to increase the level of superabsorber in absorbent cores to 70% by weight or greater. Prior to the present invention, the art had not provided a way to satisfactorily achieve this superabsorber content in absorbent cores. Addition of such high levels of superabsorber with fluffed cellulose by mixing results in settling and separation of superabsorber away from the cellulose fibers of the absorbent core fluff. Compositions in which the base cellulosic fluffing material is bonded to superabsorber, and which are subsequently disintegrated, such as in U.S. Pat. Nos. 4,444,830 and 4,424,247, do not provide the ability to include superabsorbers at greater than 50 weight percent, and further are not dense enough to be economical to ship. The art recognizes that a finished composite base fluffing/superabsorber material should have a density of greater than about 0.40 grams per cubic centimeter to allow economical shipping and distribution.
It has now been surprisingly and unexpectedly discovered by the present inventors that a composite comprising a cellulosic base fluffing material and one or more superabsorbers can be produced that comprises greater than 50% superabsorber by weight, and which is suitable for disintegration into an absorbent core material, in which the superabsorber is closely associated with or entrapped between the cellulose fluff fibers. The composite of the invention is a high density material suitable for economical transport and storage.
In one aspect, the invention relates to a three layered high density composite material comprising a layer of wetlaid cellulose, a layer of drylaid (also known as xe2x80x9cairlaidxe2x80x9d) cellulose and a layer of superabsorber disposed therebetween. The composite comprises greater than 50% by weight, and preferably greater than 70% by weight of superabsorber. The drylaid cellulose layer does not include an additional bonding agent, such as latex.
In one aspect, the layer of superabsorber is in direct contact with the wetlaid web of cellulosic fibers and the drylaid cellulosic layer is in direct contact with the superabsorber layer.
In another aspect, the invention relates to a method for producing a high density composite material suitable for disintegration into a superabsorbent fluff core material. The method comprises providing a wetlaid sheet, adding water, distributing a superabsorber layer onto the wetlaid sheet, and then providing a drylaid cellulose layer disposed adjacent to the superabsorber layer which entraps and immobilizes the superabsorber layer. The resulting three layer sheet is then lightly compressed.
In yet another aspect, the invention relates to a superabsorbent fluff material suitable for use as an absorbent core material in an absorbent article, including the superabsorber-associated fibers produced by the mechanical disintegration of the layered high density composite material of the invention.
In a further aspect, the invention relates to an absorbent article which includes the superabsorbent fluff material of the invention, and to methods for making such absorbent articles.